Chemical Profile of Kumquat (Citrus japonica var. margarita) Essential Oil, In Vitro Digestion, and Biological Activity

Kumquat is one of the smallest citrus fruits (from the Rutaceae family), and its essential oil's biological effects have not yet been sufficiently researched, in contrast to the essential oils of its relatives. Therefore, the aim of this large-scale study was to investigate the chemical profile of kumquat essential oils (KEOs) isolated by microwave-assisted distillation (MAD) and Clevenger hydrodistillation using GC-MS analysis. To test the bioaccessibility of their bioactive components, in vitro digestion with commercially available enzymes was performed. The final step of this research was to test their cytotoxic activity against a cervical cancer cell line (HeLa), a human colon cancer cell line (HCT116), a human osteosarcoma cell line (U2OS), and a healthy cell line (RPE1). Two methods were used to test the antioxidant activity: DPPH (2,2-diphenyl-1-picrylhydrazyl) and ORAC (oxygen radical absorbance capacity). The antibacterial activity was tested in relation to the growth and adhesion of <i>Escherichia coli</i> and <i>Staphylococcus aureus</i> on a polystyrene surface. The GC-MS analysis showed that the major compound in both kumquat essential oils was limonene, which was stable before and after in vitro digestion (>90%). The results showed that the cytotoxic activity of the KEOs in all three cancer cell lines tested was IC₅₀ 1-2 mg/mL, and in the healthy cell line (RPE1), the IC₅₀ value was above 4 mg/mL. The antibacterial activity of the KEOs obtained after MAD and Clevenger hydrodistillation was 4 mg/mL against <i>E. coli</i> and 1 mg/mL against <i>S. aureus</i>. The KEOs after MAD and Clevenger hydrodistillation reduced the adhesion of <i>E. coli</i> by more than 1 log, while there was no statistically significant effect on the adhesion of <i>S. aureus</i> to the polystyrene surface. Both KEOs exhibited comparable levels of antioxidant activity using both methods tested, with IC50 values of 855.25 ± 26.02 μg/mL (after MAD) and 929.41 ± 101.57 μg/mL (after Clevenger hydrodistillation) for DPPH activity and 4839.09 ± 91.99 μmol TE/g of EO (after MAD) and 4928.78 ± 275.67 μmol TE/g of EO (after Clevenger hydrodistillation) for ORAC. The results obtained show possible future applications in various fields (e.g., in the food, pharmaceutical, cosmetic, and agricultural industries).